Scrapie is a spongiform encephalopathy of sheep and goats which can be transmitted experimentally to several other animal species. Most importantly Bovine Spongiform Encephalopathy (BSE) is believed to have been acquired from sheep with scrapie. Similar diseases are recognized in humans. No etiologic agent has been identified. However, the proteinase K resistant form (PrP-res) of an endogenous protein designated prion protein (PrP) purifies with infectivity and is important to disease pathogenesis. We developed a sensitive assay for PrP-res and utilized it to diagnose scrapie in sheep. Diagnosis based on PrP-res detection was much more accurate and less subjective than the currently used method of diagnosis based on the microscopic evaluation of brain. We also showed that PrP-res analysis of spleen or lymph node was nearly as accurate as analysis of brain. We have determined that PrP-res accumulates prior to clinical disease in sheep lymph node and placenta. Thus ante-mortem diagnosis of sheep scrapie is possible. PrP-res analysis is being used to test tissues from cattle in order to determine if spongiform encephalopathy currently exists in U.S. cattle and whether an epidemic similar to BSE in Great Britain is possible in the U.S.A. So far no positive cases have been identified. PrP-res analysis should also be relevant for diagnosis of the human disease counterparts. The influence of PrP gene sequences on interspecies transmission of spongiform encephalopathies, the relative importance of neurons and astrocytes on disease pathogenesis, and the importance of peripheral lymphoid tissue on disease are also being studied. All of these investigations utilize transgenic mice which express hamster PrP (HPrP) protein. Expression of HPrP has been restricted in the mice to either neurons or astrocytes thus allowing us to study the impact of each cell type on species tropism and scrapie pathogenesis. In addition, other mice have been acquired which express no PrP at all and are being used to study the importance of PrP on disease. Information obtained from these studies should suggest cell types or tissues which might be the target of eventual therapeutic intervention.